It is narrated from Sayyiduna Anas (may Allah be pleased with him) that they consulted about something by which they could know and recognise the times of prayer. Some mentioned that they should strike a bell or light a fire. Sayyiduna Bilal (may Allah be pleased with him) was ordered to say the phrases of the adhan in pairs and the iqamah once.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1832